Photographic methods using silver halides are excellent in photographic characteristics such as sensitivity, gradation control and resolving power, as compared with other photographic methods such as electrophotographic methods and diazo photographic methods, and therefore have previously been most widely used.
At present, image information has been largely shifted from black-and-white images to color images because of a great deal of information and expressing easiness. However, black-and-white images are still preferably used in specific fields such as the medical field. Further, in the print field, plate making materials for color images are also used as black-and-white images for each printing ink. Thus, there are still great demands for the black-and-white images mainly in the industrial use.
In recent years, systems which can readily and rapidly provide images have been developed in accordance with the shift of image formation processing using light-sensitive materials containing silver halides from conventional wet processing to instant systems containing developing solutions and further to dry heat development processing by means of heating, also in view of environmental protection. Such heat developable black-and-white light-sensitive materials are described, for example, in Shashin Kohqaku no Kiso (Higinen Shashin) (The Fundamentals of Photographic Engineering (Nonsilver Photograph)), pages 242 to 255 (1982), Corona Publishing Co. Ltd., JP-B-43-4921 (the term "JP-B" as used herein means an "examined Japanese patent publication") and JP-B-43-4924. As for commercial products of the black-and-white system, "Dry Silver" light-sensitive materials are put on the market by Minnesota Mining and Manufacturing Co.
In monosheet light-sensitive materials comprising a silver halide, an organic silver salt and a reducing agent, such as Dry Silver light-sensitive materials, unused silver halide and organic silver salt remain in heat-developed images. The light-sensitive materials are therefore disadvantageous in that the residual silver halides or organic silver salts are printed out to cause coloration of white backgrounds, resulting in loss of contrast, when they are exposed to strong light or stored for a long period of time. They are further disadvantageous in that the light-sensitive materials before processing have poor storability, because a reducing agent and a silver halide coexist in the same light-sensitive material.
Further, a method for obtaining black color images by dry processing is described in Research Disclosure (hereinafter abbreviated as "RD"), No. 17326, pages 49 to 51 (September, 1978). However, this system also has the same disadvantage as described above, because it is also of unfixing type containing silver and silver salts in color images.
In order to overcome these disadvantages, JP-B-3-78617 and JP-B-3-45820 are disclose methods for forming black-and-white images, in which mobile (diffusible) dyes are imagewise formed or released by heating, and then transferred to a dye fixing material containing a dye accepting material such as mordants, a heat-resistant organic polymer, etc. using various transferring solvents, thereby improving image storability. However, in these methods, transfer is conducted after heat development, so that the number of steps is increased and the processing time is prolonged.
Further, JP-A-3-260645 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") discloses a heat development transfer type black-and-white image forming method utilizing a coupling reaction, including a method of conducting transfer after development and a method of conducting development and transfer at the same time. However, these methods also require a long period of time and a high temperature for processing because of the absence of a development transfer accelerator.
JP-A-62-129848 discloses that black-and-white images of transferred dye images can be formed by conducting heat development using a small amount of water. However, in order to obtain images having a transmission density of 2 or more, preferably 3 or more, which are required for most black-and-white images, by dye transfer methods within a short period of time, it is necessary to reduce the film thickness of a light-sensitive material, particularly to make the amount of a binder as small as possible, and to increase the amount of a dye-donating compound used. This raises the problems of lowered quality of the film and increased cost. Further, this introduces the problem that the use thereof is limited because of a reduction in sharpness due to transfer. Furthermore, it is difficult to synthesize black dye-donating compounds, and it is also difficult to obtain neutral gray color images by mixing yellow, magenta and cyan dye-donating compounds.
In addition, methods for forming silver images by a heat development silver salt diffusion transfer method using a silver halide light-sensitive material and a processing sheet are disclosed in JP-A-62-283332, JP-A-63-198050 and JP-A-60-194448. However, these methods are also methods utilizing transferred silver images. It is therefore difficult to obtain images having a transmission density of 2 or more, preferably 3 or more, and a high sharpness within a short period of time, and improvements have been required.
Further, it has become clarified that the introduction of a matting agent into a photographic element (particularly, a processing sheet) used in this image forming method for prevention of adhesion after coating and winding-up steps raises the problem of an increase in Dmin.
When the amount of a solvent for a silver halide is increased, or a solvent for a silver halide having a high stability constant with respect to silver are used for decreasing Dmin, Dmin is decreased, but Dmax is also decreased. This results from the occurrence of dissolution physical development in exposed areas, or an decrease in the amount of developed silver by the transfer of silver to a processing sheet.